1. Field of the Invention
The present invention relates generally to the field of avionics, and more particularly to a method for acquiring in situ flight data for variable flight conditions and for large frequency ranges with a reduced flight data acquisition period. That is, a range of frequency conditions are compressed such that multiple conditions can be evaluated simultaneously, reducing the time that the pilots are exposed to flight conditions on the envelope of the plane""s operable capability.
2. Description of Related Art
Most modern tactical aircraft are designed to be aerodynamically unstable, or at least marginally stable during flight. Two examples of unstable aircraft are the F-117 and the F-16. For these types of aircraft, it is critical to know the exact transfer function of the forces generated by the thrust system for a wide range of flight conditions. The historic method for generating the transfer function is known as a xe2x80x9cfrequency sweep.xe2x80x9d A frequency sweep is a series of in situ maneuvers made by a pilot in an aircraft while flight data is recorded. For example, the F-15 ACTIVE was a fly-by-wire controlled aircraft with an experimental thrust vectoring system. Large movable or variable geometry nozzles were used to vector the engine thrust in a given direction. Once it was determined that the plane could be redirected by moving the nozzles, the next step was to map the response of the plane to various xe2x80x9cstickxe2x80x9d maneuvers. Here, the xe2x80x9cstickxe2x80x9d refers to the controls for the nozzles as the nozzles are moved back and forth.
The nozzles have a mechanical limitation as to how quickly the nozzles can be rotatedxe2x80x94in the present example the nozzle was limited to a maximum angular rate change of 80 degrees per second. Thus, if an eight degree change was called for, the shortest time required for this change was one tenth of a second. To generate a frequency sweep, the pilot would climb the aircraft to its designated flying altitude and level off, and turn on the flight data recording instruments. The pilot would then manually move the stick in a sinusoidal manner, starting by pushing the stick incrementally forward slowly and then back. The pilot would repeat the stick movements, gradually increasing in speed as the plane responded to the various control inputs. This process is continued until either the maximum gimbal rate is achieved, or until the aircraft response can longer keep up with the flight control inputs. The latter condition is indicative that the input frequency is higher than the maximum frequency that the aircraft can respond to. This entire process may take as long as sixty to ninety seconds or more, depending on the various flight conditions. In evaluating the performance characteristics of the aircraft, the frequency response and the maximum input frequency are critical values.
The advent of computer controlled flight relieved the pilot of the duty of manually controlling the stick maneuvers, and replaced the pilot with a computer program. In other words, a computer would be programmed to deliver the aircraft through a series of predetermined maneuvers, or waveform, and the flight data would be recorded in response to the computer controlled waveform. However, the problem still remained that a set of flight data for a given condition could require a minute or more of in situ flight frequency sweeps (sine wave with varying frequency) to obtain enough power content over the desired frequency range. Identifying transfer functions for standard aerodynamic surfaces (ailerons, rudders, etc.) was possible because these surfaces are relatively unaffected by throttle changes. Consequently, the throttles could be set to maintain flight condition and the plane would remain at a specified flight condition for as long as required. The characteristics of thrust vectoring, however, are greatly affected by throttle position. Thus, to obtain a complete transfer function map of the thrust vectoring used for the particular application, tests at many power settings from idle to maximum thrust were necessary. The further deviations from level flight throttle setting caused the aircraft to accelerate and thereby escape from the desired flight condition. For tests at extremes from the level flight condition, the sixty seconds or more of flight data was unacceptably long. There existed a need to obtain a flight response to variable conditions in a shortened time, conserving flight times and provide more data efficiently in as little in situ flight time as possible.
In order to obtain the desired flight test data, a method had to be derived to command the thrust vectoring with as much frequency content as possible in as short a period as possibly while operating within the mechanical constraints of the aircraft nozzle. In place was a programmable test input (PTI) system which was used to command open-loop surface excitations on the F-15 ACTIVE. This system allowed predetermined sequences of up to five sine or piece-wise-linear waveforms to be calculated real-time and commanded to any aircraft control effector during single command sequence. The sine waveforms could vary frequency over time to perform frequency sweeps. The five waveforms are referred to as a xe2x80x9cdataset.xe2x80x9d However, the PTI system provided for only one dataset to be executed at a time.
The present invention includes an algorithm that generates a waveform consisting of an arbitrary number of frequency sweeps combined from adding and subtracting obtained frequency sweeps. Optimization routines determine the best combination order of frequency sweeps to minimize the maximum deflection or maximum command rate of the wave form. The algorithm allows for arbitrary output timing, or commands per second issued for the desired waveform, arbitrary starting and ending frequencies and amplitudes, arbitrary number of frequency sweep components, arbitrary frequency sweep exponent, arbitrary amplitude sweep exponent, and arbitrary waveform length.
For a given frequency range and sweep exponent, amplitude range and sweep exponent, desired total waveform time an number of frequency sweep components, the algorithm can determine the optimum arrangement of the components to minimize the maximum amplitude or rate. Using the above input information, the starting and ending frequencies and amplitudes are calculated for each frequency sweep component (stack) along with the amplitude and frequency sweep exponents. Each stack is then calculated and stored in a matrix. Once all stacks are calculated, all combinations of adding and subtracting the stacks are performed. The maximum amplitude and rate are recorded in a vector for each combination. Once all combinations are calculated, the frequency and amplitude ranges and sweep exponents for the final waveform are provided as output. The final waveform is recalculated and also provided as output.